Add support for union to enum

[miku1958]

Description

Add basic support to convert string/number literal union to enum, for example:

interface FullSize extends BaseSize, CustomSize {
  stringUnion1: 'A1' | 'B1';
  numUnion1: 11 | 21;
}

⬇️

public struct FullSize: Codable {
  public var stringUnion1: FullSizeStringUnion1
  public var numUnion1: FullSizeNumUnion1
}

public enum FullSizeStringUnion1: String, Codable {
  case a1 = "A1"
  case b1 = "B1"
}

public enum FullSizeNumUnion1: Int, Codable {
  case _11 = 11
  case _21 = 21
}

Known issues

Since mustache does not allow to nest mustache, it will lead to a dead loop, so it can't be simply/neatly converted to:

public struct FullSize: Codable {
  public enum StringUnion1: String, Codable {
    case a1 = "A1"
    case b1 = "B1"
  }

  public var stringUnion1: tringUnion1
}

For simplicity of implementation, it is generated at the outermost level.

[zhuorantan]

Can you update the documentation to reflect the change of behaviours in this PR?